SimScale CAE Forum

Aerodynamics in passenger car


#1

Hi.
I’m new in CFD and need some help before I will make wrong assumptions.
I’m driving my cars without a rear spoiler, but since I will be mounting it soon I was curious if there are any benefits of a stock spoiler except better look.
I found some 3D project of car I’m using IRL, and decided to make some tests at 125kph for a general overview.
First case is stock plain rear end.


Second one is small lip on rear.

And the last one is stock spoiler

If I’m correct “nut” in case without spoiler is sucking rear side of car causing drag?
Smallest “nut” is on version with tiny lip, but it creates a jet of air striking upward and more dirty air at the bottom for a car behind, while stock spoiler looks like it creates more steady airflow behind. Which one of those two spoilers helps better eliminate drag and could be more useful on track use?
Here’s the link for a project-> Project


#2

hi @pszczelaszkov,
your stock spoiler and small lip don’t make differences because both generate height low-pressure zone at the back of your car and a larger wake was generated. remember spoiler work at only high speed and use for generated large downforce. in your case spoiler are flat i think its should be like aerofoil for high downforce.

if your aim was to eliminated drag only then i think the stock plain rear end was good.

thank you,
Rohit.


#3

@ROHIT_SR
Thanks for response, actually as reference I was using this blog entry.


Was focused on this part

  • Spoilers are barricades to undesirable flows, and thus are able to reshape airflow streams around the vehicle. This can help keep the rear of the vehicle down and decrease drag by changing the effective vehicle shape.

Clearly those two do something with air but, are they making more good or bad things with air behind?
Although sedan rear end is not so tragic as coupe.


#4

hi @pszczelaszkov,
thank you for this link. really some good information about aerodynamic(wing and spoiler) :star_struck:
#redbull_gives_you_wings_NOT_SPOILERS. :joy::raised_hands:
ROHIT.


#5

Hi @pszczelaszkov,

The answer is not straightforward. Spoilers on top of generating downforce, also generate drag no matter what. The issue is whether the influence of the spoiler will help in reducing the overall drag of the vehicle. If you’re talking just a spoiler itself, the net gain in drag will always be positive regardless of how well designed your spoiler is. You can’t expect a device that manipulates the flow of air to not have drag.

While the specifics of how to control the flow of air with specific spoiler designs in order to reduce drag is not something I am familiar with, off the top of my head I would assume that the key concept is to ensure that flow manipulated by the spoiler is of minimal turbulence. Turbulent flow typically contributes significantly to drag, the spoiler helps reduce that, hence overall drag is reduced. I’m sure I am also missing other things like the influence on the rear wake of the car or something like that. @ROHIT_SR is much more experienced in these concepts. Maybe he can share some insights.

Cheers.

Regards,
Barry


#6

Hello again @ROHIT_SR.
Since you were talking about pressure and the wage I decided to change some fields for better understanding pressure rather than “nut” which is as I suspect an acceleration of particles(m^2/s)?
Also i put seed on the rear, just noticed, that should be a correct way.

For nonspoiler pressure and particles swirl looks like this.

For lip version i have

and final stock spoiler looks like this

Can we predict according to shoots of swirls and data from “nut” which one is more turbulent?
Also take a look at wage’s at non spoiler and stock spoiler. They are not so different now, while lip is creating some underpressure pocket.
I tried to generate drag coefficient chart but I failed, im getting negative values, probably because of mesh and i dont know if i can trust those values. Maybe for sake of comparision they can be useful?


#7

hi @pszczelaszkov,

all post-processing image gives the answer to your question.

  1. non-spoiler create the turbulent almost lower side of the car. this kind of turbulent by the lower part of the car.when flow separated from the car part(just like a diffuser.)

if you notice both cases create different turbulence. in a spoiler case has two turbulent zones.first, one generated by spoiler itself and the second one was the underbody part.
more you restrict the flow more turbulent was created.


your spoiler has high turbulent (high wake ) and also have a high low-pressure zone.

i agreed with the @Get_Barried comments.

can you explain step by step procedure for a cd.?

Thank You,
Rohit.


#8

Hi @ROHIT_SR
Since velocity is Y=-35m/s and roof is on +Z. I setup drag vector at Y=1 and lift vector at Z=1
Which gives me respectively nonspoiler, lip and spoiler Drag coefficients as follows:
-0.94,-1.02,-0.98
Lift coeficients:
0.017,-0.49,-0.29
Pressure forces Z:
11,-367,-215
I think I can trust on lift values since it should work for the whole body anyway.
But drag is messed up, I think it’s because of large open space in mesh, and spoiler sharing same face as rest of the body. However I can’t separate spoiler from hull on this geometry, it seems the only way it works is by importing as STL. When I’m trying with STEP, meshing process is ignoring open bodies leaving me with decals. I think i will recreate geometry from a blueprint as a more solid block, maybe then STEP will work and let me separate drag modifiers from rest of the body.
At this point, if I’m about to believe in lift values it seems like lip spoiler more likely helps push body towards ground making it more stable at high velocities. But I’m still curious about drag changes just for sake of clarity of results.
Please take a look at the modification section:
http://www.iosrjournals.org/iosr-jmce/papers/vol13-issue3/Version-1/R130301114118.pdf
There is something im missing, maybe its a brain :thinking:
I think i will come back with new geometry.
Cheers,
pszczelaszkov